Method of separating rare earths



United States Patent METHOD OF SEPARATING RARE EARTHS Frank H. Spedding, Earl J. Wheelwright, and Jack E. Powell, Ames, Iowa, assignors to the United States of America as represented by the United States Atomic Energy Commission N0 Drawing. Application February 11, 1954, Serial No. 409,783

19 Claims. (Cl. 2-3-19) This invention is concerned with a method for the fractionation of rare earth mixtures and the recovery of rare earths.

The term rare earths as it is used in the present specification includes the lanthanide rare earth elements having atomic numbers from 57 to 71, inclusive, and the element yttrium, atomic number 39, which is ordinarily found in rare earth concentrates and acts similarly to the rare earths in chemical separations.

The rare earths are usually obtained from ore concentrates of the oxides. Until very recent years these rare earth oxide concentrates were separated into their indi vidual components by the method of fractional crystallization. Because of the great chemical similarity of the rare earths this fractional crystallization procedure was a very laborious task and it was extremely difficult to obtain even comparatively pure individual rare earth species by this method.

A major advance in the recovery of rare earths occurred recently with the discovery of the ion exchange method of obtaining individual rare earth species. In the citrate ion exchange process oxides of the ore concentrates are dissolved in a slight excess of an aqueous acid and the resultant solution is passed through a cation exchange resin column whereby the rare earths are deposited in a band on the resin. A buffered citric acid solution is then passed through the column whereby the individual rare earths are formed into bands which pass down the column. The separation of the individual species into separate bands is a result of the species coming to equilibrium with the eluate. The concentrations of the various ions in the eluate automatically adjust themselves as determined by the stability constants of the various rare earth-citrate complexes, and therefore one band succeeds another as they come oif the column.

While this method yields very pure individual rare earth species, it does have several disadvantages. The first disadvantage is that the individual rare earths are present in the concentrates in widely varying concentrations. Some of the rare earths are present in very small amounts and others, such as cerium, lanthanum, neo- 2,798,789 Patented July 9, 1957 constants of the individual species with the citrate complexes. These equilibrium constants are affected by the number of bonds of the rare earths with the citrate ion, the bond strength and the rare earth ionic size. The two citrate anions associated with the rare earths are tridentate complexing agents, so it is apparent that these agents are not as sensitive to ion size as would be an agent having more available bonds for attachment to the rare earths. Longer columns are therefore required to attain equilibrium conditions for the citrate complexing agent than would be the case for a complexing agent having a larger number of bonds.

It is an object of the present invention to provide a method of fractionating a mixture of rare earths, such as are found in ore concentrates, into fractions which contain only a few adjacent rare earths.

It is an additional object of the present invention to provide a method of separating rare earth mixtures into their individual components.

A further object of the present invention is to provide a method for the recovery of individual rare earth species from rare earth concentrates containing a plurality of rare earth species.

Still further objects will be apparent from the description of the present invention which follows.

Ethylenediamine tetraacetic acid is a hexadentate complexing agent which forms soluble complexes with all rare earths. The stability constants of the rare earthethylenediamine tetraacetic acid (or E. D. T. A.) complexes have been measured; they vary from 10 for lanthanum to 10 for lutetium. The stability constant of yttrium is 10 so that it falls between terbium and dysprosium. Although the stability constants of the rare earth-citrate complexes have not been measured, it is almost certain that the citrate-rare earth complexes are much less strongly bound than are the rare earth E. D. T. A. complexes and that the variation between the stability constants of the individual rare earth-citrate complexes is much less than that between the individual rare earth-E. D. T. A. complexes. In spite of this more favorable difference of stability constants between individual rare earths, however, E. D. T. A. cannot be used as an elution agent in the same manner as the citric acids are used. For one thing, E. D. T. A. precipitates in media having a pH of less than about 2; hence no part of the resin bed through which the E. D. T. A. passes can be in the acid cycle. Furthermore, the rare earth- E. D. T. A. stability constants are greater by many orders of magnitude than the stability constants of P-, Na-. or NH-E. D. T. A., so that, if a solution of rare earth- E. D. T. A. complex is passed through a cation resin column in the hydrogen, sodium or ammonium form, there will be little or no exchange of cations between the complex and the resin. Since it is the selective adsorption of the included cations in the bed during the course of passage of the eluant through the bed which causes the separation into species, the passage of the rare earth E. D. T. A. complex through the resin bed would not result in a separation.

It has been found that mixtures of rare earth species may be fractionated into several fractions, each fraction containing a small number of rare earths. The rare earths of the mixture are dissolved in an aqueous acid and the excess hydrogen ions are then reduced by adding ammonium hydroxide to the solution until the pH of the solution is about 5'0'1" greater. This solution of mixed rare earths is then contacted with a quantity of an ethylenediamine tetraacetic acid reagent stoichiometrically insufiicient to complex all of the rare earths present under alkaline or slightly acid conditions. The heavy rare earths form a more stable complex with E. D. T. A. than the lighter rare earths so the heavier rare earths will be preferentially complexed. The solution is permitted to approach equilibrium and then brought into contact with a cation exchange resin which adsorbs the uncomplexed rare earths, leaving as the first fraction the coniplexed rare earths dissolved in the aqueous medium. Additional rare earth fractions may be obtained by contacting the resin containing the adsorbed rare earths with successive quantities of E. D. T. A. solution, each sufiicient to complex and thereby remove from the resin only a part of the rare earths adsorbed on the resin. This method thus separates a complex rare earth mixture into fractions which contain only a few adjacent rare earths.

It has further been found that, in accordance with the process of this invention, these fractions can be separated into the individual species by using E. D. T. A. as an eluting agent. Although the stability constants of the complexes of the rare earths with E. D. T. A. are much greater than with the cations norm-ally present in an ion exchange resin, for example sodium or ammonium, there are several metals which form more stable complexes with E. D. T. \A. than do substantially any of the rare earths. lron and copper are two of these metals. Based upon this fact a procedure has been developed which permits the use of E. D. T. A. as an elution agent for rare earths. In accordance with this procedure the rare earths are first adsorbed on a cation exchange resin bed in the conventional manner. This bed containing the adsorbed rare earths is then physically connected with a second cation exchange resin bed containing a cation exchange resin in the ferric or cupric cycle. The ferric or cupric ion may be considered to be a retaining ion. An aqueous E. D. T. A. solution is then passed in sequence through the first bed containing the adsorbed rare earths and then through the second bed containing the resin in the ferric or cupric cycle. The rare earths are eluted from the first resin bed in the form of rare earth-E. D. F1". A. complexes and these complexes then exchange with the ferric or cupric ions in the second resin bed. The resultant iron -E. D. T. A. or c-opper E. D. T. A. complexes are water-soluble and are swept out of the second column. The rare earth species form individual bands in the second column and these bands pass through the column, resulting in a very effective separation of the rare earths into the individual species as elution takes place in the column.

The :first step of the present process is concerned with the fractionation of the mixed rare earths into separate tract-ions containing a smaller number of rare earths. It the rare earth mixture is in the usual form of the rare earth ore concentrate, consisting of rare earth oxides of most of the rare earths including yttrium, these oxides are dissolved in an aqueous acidic solution, and excess of hydrogen ions is neutralized with a suitable base so that the resultant solution has a pH of about 5 or greater. Alternatively the oxides are converted to water-soluble salts such as the chlorides and these salts are then dissolved in water. It is then necessary to evaluate the rare earths present and to determine the number of fractions into which it is desired to separate the rare earths. Evaluation may be done by analysis, or, since precise evaluation is not required in this step, an assumption of the relative percentage =of different rare earths present may be made on the basis of past analysis of the particular ore from which the concentrate is obtained. The stability constants, or equilibrium constants, of the complexes formed between the rare earth metal ions and the anions of E. D. T. A. have been measured and are set forth in Table I. The method ofmeasurement is described in the J. A. C. S. 75, 4196 4 Table I Metal M log KMY l4. 725:0. 06 15. 395-0. 06 15. 755:0. 06 10. 60i0. 06 16. 555:0. 07 16. 695:0. 08 16. i0. 08 17. 385:0. 10 17. 565:0. 08 17. 755:0. 08 18. 315:0. 07 18. 555:0. 07 19. 075:0. 08 19. 395:0. 10 19. 655:0. 12

The equilibrium constant is defined as where M indicates rare earth cation and '1 indicates E. :D. T. A. anion. The stoichiometric amount of E. D. T. A. required to complex the portion of rare earths present which are to be separated into each fraction can be determined from the equation KMY acetic acid agent is used in the present specification and claims to designate the acid and the readily ionizable salts of the acid.

The complexing of a portion of the rare earths may take place in a tank or other suitable type of reactor. For maximum efliciency the E. D. T. A. should be maintained in contact with the resin for a suflicient time to come to, or close to, equilibrium. The pH of the mixture of E. D. T. A. and rare earths should be maintained greater than approximately 2, and preferably between 4 and 9, since E. D. T. A. tends to precipitate in strongly acid solutions.

Following the equilibration of the E. D. T. A. and rare earths solution, the complexed rare earth fraction is separated from the uncomplcxed rare earths by contacting the entire mixture with a cation exchange resin. This may be accomplished by equilibration of the resin and E. D. T. A.-rare earth mixture in a tank-type reactor, but it is preferably accomplished by passing the E. D. T. A.- rare earth solution through a bed of the resin.

Cation exchange resins capable of absorbing rare earth cations are those of the phenolic methylene sulfonic type, of the nuclear sulfonic type and of the carboxylic type. In order to obtain full separation of the complexed fraction from the uncomplexed rare earths, it is important that the amount of resin used be sufficient to adequately adsorb all of the uncomplexed rare earths.

The efiluent from the column will contain the complexed fraction of the rare earths. These rare earths may be recovered from the E. D. T. A. complex by a metathesis" with ferric or cupric ion, by decomposing the complex by heat, by an oxalate precipitation, or by other similar methods. The rare earths which are absorbed on the resin may be removed from the resin by elution in a single pass, or further fractionation may be carried out. If it is desired to carry out further fractionation, an cluant containing a limited amount of E. D. T. A. sufficie'nt to complex only a part of the adsorbed rare earths is passed through the column. Additional fractions may be then removed from the column in a similar manner.

Now that the fractionation step of our process has been generally described, it may be further illustrated by the following specific examples.

EXAMPLE I A neutral solution containing 312 grams of mixed rare earths, weighed as the oxide, was obtained by dissolving a known larger amount of the rare earth material in a limited amount of hydrochloric acid and recovering the undissolved rare earths. It was determined that the rare earth solution contained about 60% yttrium, heavy rare earths (lutetium, ytterbium, thulium, erbium, holmium and dysprosium) and 25% light rare earths (terbium, gadolinium, samarium, neodymium, praseodymium, cerium and lanthanum). It was decided to complex the heavy rare earths, lutetium to dysprosium, to make the initial separation with an ion exchange column, and then to remove the yttrium and the lighter rare earths from the column in several fractions by passing successive solutions containing a predetermined amount of E. D. T. A. through the column. The amount of E. D. T. A. agent required to complex the heavy rare earths. present was then calculated and found to be approximately 0.23 mole or 75 grams of the dihydrogendiammonium ethylenediamine tetraacetic acid. This quantity of E. D. T. A. agent was dissolved in water, adjusted to a pH of 10 with ammonium hydroxide and then added to the rare earth solution; the combined solution was diluted to 90 liters. After equilibration for 24 hours the pH of the solution was adjusted to 4.5 with a little hydrochloric acid. Following a second 24-hour equilibration period, this combined solution Was passed rapidly through a short bed of the hydrocarbon-polymertype cation exchange resin containing nuclear sulfonic acid groups prepared in accordance with the method described in U. S. Patent 2,366,007. The column was then washed free of complexed rare earths with a few liters of distilled water and this Wash was combined with the effiuent from the adsorption cycle to form the first fraction. Four successive fractions were then removed from the column by pouring four successive predetermined amounts of E. D. T. A. solution, which had been adjusted to a pH of 9.5, through the column at a slow flow rate, taking 24 hours for each fraction, so as to let the liquid remain in contact with the resin long enough to approach equilibrium conditions. Of these four E. D. T. A. eluants, the first two each contained 273 grams of the dihydrogen-diammonium form of E. D. T. A. and the second two 120 grams each. The five fractions were then analyzed and the analysis is tabulated in Table II. Sample I is the eflluent from the adsorption cycle and the water wash. Samples II and III are the 273- gram E. D. T. A. eluants and samples IV and V are the 120-gram E. D. T. A. eluants.

Table II Rare Earth Oxides Recovered in One Run on Gadollnite Ore (Grams) same manner.

While the conditions under which the example was carried out did not result in absolute separation of the different species in the various fractions, it will be noticed that most of the heavy rare earths are concentrated away from the light rare earths and from most of the yttrium. Further improvements in the fractionation may be achieved by operating at a temperature higher than room temperature so as to approach the various equilibria more rapidly and by utilizing more concentrated solutions to increase the yields. Further purification may also be obtained by additional fractionations.

The fractions obtained by this method may then be separated into the individual species by the use of an E. D. T. A. elution step in accordance with the further method of the present invention, as will be hereinafter described, or the species may be separated by conventional methods, such as the citrate elution method.

The separation of a fraction into individual species by the process of this invention first requires that the fraction be adsorbed on a resin column. A preferable method of accomplishing this comprises as a first step the conversion of the rare earth-E. D. T. A. complexes to watersoluble salts, such as the chloride. The rare earth chloride fraction is then adsorbed upon a cation exchange resin column in the sodium or ammonium cycle by conventional methods. A hydrogen-state-resin bed can also be used since a neutral solution of rare earth ions (pH 5 to 6) will displace essentially all of the hydrogen ions from the resin bed. This column containing the adsorbed rare earths is then physically connected with a column containing a cation exchange resin which has been converted to the ferric cycle. The rare earths are then eluted through the ferric cycle resin. The ferric cycle resin may be prepared by passing an aqueous solution of a ferric salt, such as ferric nitrate, through a cation exchange resin column. The stability constant of the E. D. T. A.-iron complex is 10 much larger than any of the rare earth-E. D. T. A. complex constants; consequently during the elution process, when the rare earth complex reaches the iron phase resin, an exchange takes place and the rare earth is readsorbed. Since the iron complex is water-soluble, it is swept out of the column. The various rare earth bands evidently operate in the As the less tightly held rare earth complex comes in contact with the rare earth ion which forms a more stable complex, an exchange takes place. Gradually with repeated exchanges the concentrations of the heavier rare earths increase and approach in the individual leading bands, while the concentrations of the lighter rare earths increase and approach 100% in the individual trailing bands. Thus, by passing an E. D. T. A. solution through the column and collecting the efiluent in separate portions, portions can be obtained which contain substantially pure rare earth species.

The resin used in this step of the process may be a commercial cation exchange resin of the types described in the previous modification. Since E. D. T. A. tends to precipitate in strongly acid solutions, it is essential that the E. D. T. A. solutions be maintained at a pH greater than about 2, and preferably between about 4 and 9, during all steps of the exchange.

While the Fe -E. D. T. A. stability constant is greater than that of any rare earth-E. D. T. A. complex, there are other metallic cations, such as copper, nickel and lead which form complexes the stability constants of which are greater than about 10 The stability constants of these complexes are greater than those of most of the rare earth-E. D. T. A. complexes. In principle, if the E. D. T. A. complex of the cation of the second resin bed has a stability constant greater than that of any rare earth-E. D. T. A. complex of a group of rare earths to be separated, effective separations of that group of rare earths by the method of the present invention with the second resin bed in that cation cycle should be possible. The practical usefulness of some of these cations :m y .1 limited, however. should form an insoluble precipitate ,in the pH range Forexample, if the cation which must be used to prevent precipitation of-the E. D. T. A., then the usefulness of that cation would belimited.

The relative stability constantsofthe E. D. T. A.,complexes is not the only factor that must be considered in the determination of the order of elution of cationic species from cation exchange resins, however. The degree of attraction between the resin and the ion is also dependent upon the charge of the ion. The higher the charge on the cation, the greater is the afiinity of the ion for the resin. Thus a trivalent ion is more tightly bound to a cation exchange resin than a divalent ion. Therefore, while the above-mentioned divalent ions are complexed less strongly by E. D. T. A. than lutetium and the heavier rare earths, they are also less tightly bound to the resin than any of the trivalent rare earths. This affinity effect therefore tends to favor the elution of these divalent ions such as copper from a cation exchange resin by E. D. T. A. over the elution of the heavier rare earths of the series. This shift, under normal circumstances, causes copper to be eluted even before lutetium from a cation exchange bed when E. D. T. A. is used as the eluant. The use of a second resin bed in the copper cycle is particularly desirable since copper does not form a hydroxide precipitate in the pH range used in the presentinven-tion. Therefore copper will elute ahead of all of the rare earths and does not trail into the rare earth eluate fraction. In general it may be said that when the process of the present invention is used to separate any mixture of rare earths, the retaining ion of the second resin bed can be any cation which will elute ahead of all of the rare earths as the rare earths progress down the ion exchange column.

EXAMPLE II lnthe present example a first resin bed completely saturated with a rare earth mixture was connected in series with a second resin bed in the copper state. An aqueous solution of rare earth chloride was used to saturate the first resin bed and a solution of cupric sulfate was used to saturate the second resin bed. Both beds were rinsed free of unadsorbed cations with distilled wa- 'ter prior to eluting in series with a solution of ammonium ethylenediamine tetraacetate. The rare earths which were picked up by the E. D. T. A. solution in the first column were displaced from the E. D. T. A. complex by copper ion in the second column and forced to redeposit on the second resin bed. The eflluent solution was collected after the rare earth breakthrough occurred and any copper present was eliminated by electrolysis after the solution was made acidic with sulfuric acid. The rare earths present were recovered by the addition of oxalic acid to the copper-free solution.

The rare earth mixture consisted of 53.0 grams of R203 (where R designates the rare earths generically) containing 53.0% LuzOs, 33.0% YbaOs. 11.9% Tmz'Os, 2.1% EraOa, and traces of other rare earths, dissolved in a minimum of hydrochloric acid. The resin selected for both beds was a nuclear sulfonic acid type resin. The rare earths were adsorbed on a first bed of the resin having a grain size of |50 mesh, the bed was 90 cm. long and 22 mm. in diameter. The rare earths were then eluted through a second resin bed, 90 cm. long and 22 mm. in diameter, and packed with a resin having a particle size of -100+200 mesh. The eluant was a solution of approximately 2% E. D. T. A. It had been prepared by dissolving 900 grams of diammonium-dihydrogen ethylenedialnine tetraacetate in 300 ml. of concentrated ammonium hydroxide (15 M) and liters of distilled water. A linear flow rate of 0.5 cm. per minute was maintained. The copper ions were substantially displaced from the second resin bed before even the heaviest'rare earths, and except for slight channeling effects the copper-rare earth boundary was sharp. Less than :400 ml. of eluate contained-both copper and the rare earths. The analysis of the mdivldual samples of eluate is shown 1n Table III.

Table III Fraction Total Vo1- Weight Percent Percent Percent Percent Number time, 1 R 03, g 1.11 03 I Yb Oa T111203 Ergo;

4. 6 0. 4036 99. 8 0.2 4. 8 l. 2157 99. 5 0. 5 5.0 2. 2398 99.0 1.0 5. 2 2. 2642 99:0 1. 0 5. 4 2. 2613 97. 5 2. 5 5. 6 2. 2755 97. 5 2. 5 5. 8 2. 3111 95. 5 4. 5 6. O 2. 3027 92. 1 7. 9 6. 2 2. 2936 88. 1 11. 9 6. 4 2. 2746 82. 7 17. 3 6. 6 2. 2530 77. 5 22. 5 6. 8 2. 1256 75.6 24. 4 7. 0 2. 1822 73. 7 26. 3 7. 2 2. 1243 69. 6 30. 4 7. 4 2. 2149 57. 1 42. 9 7. 6 2. 1959 28. 9 71. 1 7. s 2, 1914 7. 2 92. s 8.0 2.1102 3. 5 96. 5 8. 2 2. 2627 0. 5 99. 5 S. 4 2. 3058 0. 2 99. 5 8. G 2. 3037 0. 1 76. 2 8. 8 14.0 9. 0 1. 0 9. 2 9. 4

Similar successful runs were carried out with other groups of rare earths, such as dysprosium-yttrium-terbium and gadolinium-europinm-samarium, which are difficult to separate by conventional methods.

The process of the present invention is further illustrated by the following example.

EXAMPLE III A rare earth chloride solution was prepared by dissolving equal amounts of PrsOn and NdzOs in a slight excess of HCl. The pH of the solution was adjusted to 5 by the addition of NH4OH and the solution was passed through a small column containing a nuclear-sulfonicacid-type resin. The amount of solution was sufiicient to completely saturate the resin with rare earths. The amount of rare earth which was adsorbed on the resin was determined by the difference between that originally dissolved and that which passed through the column. A second resin column of similar diameter was then prepared. This column contained more resin than the fisrt column and had been saturated with ferric ion by passing a solution of Fe(NOs)3 through it. This bed had then been washed with water to remove the ammonium and nitrate ions. The bottom of the first column was connected to the top of the second column and the system was eluted with a solution containing 10 grams of the diammonium-dihydrogen salt of E. D. T. A. per liter. The solution had been adjusted to a pH of 8.04 with ammonium hydroxide. The flow rate of the eluant was fixed at 50 ml. per hour and the eluate was collected in ZOO-ml. fractions. The rare earth content of each fraction was precipitated with oxalic acid, and the precipitate was filtered, washed, and ignited to the oxide. These precipitates were redissolved and analyzed spectrophotometrically to determine the individual rare earth contents. Table IV shows the analyses of the nine fractions obtained.

Table IV Sample No.

While preferred embodiments of the present invention are described above, other modifications may be made without departing from the spirit and scope of the inven tion. Thus, for example, while the fractionation process of the first part of the present invention is particularly suitable to the fractionation of ore concentrates, it may be applied to mixtures of rare earths obtained from other sources and, while the separation of rare earths of the second step is particularly applicable to the separation of rare earths contained in fractions obtained by the method of the first step, it may be also applied to the separation of any mixture of rare earths. Thus this invention is not to be limited to the examples, but it is to be limited only by the appended claims.

What is claimed is:

1. A process of separating a plurality of rare earth metal values contained in an aqueous solution from each other, comprising contacting said solution with a nonacidic cation exchange resin whereby said rare earth metal values are adsorbed on said resin; contacting said resin with an aqueous solution of an ethylenediamine tetraacetic acid agent having a pH value of at least for a sufiiciently long period of time to allow equilibration, the amount of said ethylenediamine tetraacetic acid agent being less than the stoichiometric amount required to complex all of the rare earths adsorbed on said resin; and separating the resin containing the noncomplexed rare earth values from the solution containing the complexed rare earth values.

2. The process of claim 1 wherein the ethylenediamine tetraacetic acid agent is ethylenediamine tetraacetic acid.

3. The process of claim 1 wherein the ethylenediamine tetraacetic acid agent is sodium ethylenediamine tetraacetate.

4. The process of claim 1 wherein the ethylenediamine tetraacetic acid agent is ammonium ethylenediamine tetraacetate.

5. The process of claim 1 wherein the ethylenediamine tetraacetic acid agent is dihydrogen diammonium ethylenediamine tetraacetate.

6. The process of claim 1 in which the cycle consisting of contacting the resin with the ethylenediamine tetraacetic acid agent and the resin-solution separation is repeated until all rare earth values have been complexed and separated from the resin and the solutions of each cycle containing the complexed rare earth Values are separately collected.

7. A process of separating a plurality of rare earth metal values contained in an aqueous solution from each other, comprising providing a pH value of at least 5 in said aqueous solution; adding an aqueous solution of an ethylenediamine tetraacetic acid agent in an amount less than the stoichiometric amount required to complex all the rare earths, whereby the heavy rare earths are complexed while the lighter ones remain noncomplexed, said amount depending on the degree of complexing to be desired; establishing a pH value of at least 2 in the rare earths-ethylenediamine tetraacetic mixture; letting said mixture stand for several hours for equilibration; contacting said mixture with a cation exchange resin in a nonacidic form for a period of time suflicient to reach equilibrium at which the noncomplexed rare earth values are adsorbed on said resin; and separating said resin from said solution, the latter containing the heavy complexed rare earths.

8. The process of claim 7 wherein the pH value of said mixture ranges between 4 and 9.

9. The process of claim 7 wherein the ethylenediamine tetraacetic acid agent is dihydrogen diammonium ethylenediamine tetraacetate.

10. The process of claim 7 wherein the ethylenediamine tetraacetic acid agent is ethylenediamine tetraacetic acid.

11. The process of claim 7 wherein the ethylenediamine tetraacetic acid agent is sodium ethylenediamine tetraacetate.

12. The process of claim 7 wherein said resin containing said noncomplexed rare earth metal values is contacted with an aqueous solution of an ethylenediamine tetraacetic acid agent having a pH value of at least 5 for a sufficiently long period of time to allow equilibration, the amount of said ethylenediamine tetraacetic acid agent being less than the stoichiometric amount required to complex all of the rare earths adsorbed on said resin; and separating the resin containing the noncomplexed rare earth values from the solution containing the complexed rare earth values.

13. The process of claim 7 wherein the heavy complexed rare earths in said solution, after separation from the resin containing the light noncomplexed rare earths, are decomplexed; contacting said decomplexed solution with a nonacidic cation exchange resin whereby said heavy rare earths are adsorbed on said resin; contacting said resin with an aqueous solution of an ethylenediamine tetraacetic acid agent having a pH value of a least 5 for a sufiiciently long period of time to allow equilibration, the amount of said ethylenediamine tetraacetic acid agent being less than the stoichiometric amount required to complex all of the heavy rare earths adsorbed on said resin; and separating the resin containing the noncomplexed rare earth values from the solution containing the complexed rare earth values.

14. The process of claim 13 wherein decomplexing is accomplished by heating.

15. The process of claim 13 wherein decomplexing is accomplished by reaction with a ferric salt.

16. The process of claim 13 wherein decomplexing is accomplished by reaction with a cupric salt.

17. The process of claim 13 wherein decomplexing is accomplished by reaction with oxalic acid anions whereby a precipitate is formed and the precipitate is then dissolved.

18. The process of claim 7 wherein said solution containing the heavy complexed rare earths is passed over a metal salt of a cation exchange resin slowly enough to establish equilibrium whereby the rare earths are decomplexed in the order of decreasing atomic Weight and held by the resin and the corresponding metal ethylenediamine tetraacetic complexes are formed, dissolved by the solution and removed from the resin in the form of an efiluent in the order of decomplexing, said metal salt being selected from the group consisting of copper (II) salt, nickle (II) salt, lead (II) salt, and iron (III) salt and mixtures thereof; and said efiluent is then collected in fractions.

19. A method of desorbing and simultaneouslyfractionating a plurality of rare earth metal values adsorbed on a cation exchange resin, comprising washing said cation exchange resin with an aqueous solution of an ethylenediamine tetraacetic acid agent having a pH value of at least 5 whereby all rare earths are complexed, dissolved in said ethylenediamine tetraacetic solution and removed from said resin; flowing said rare earths-containing ethylenediamine tetraacetic solution over a metal salt of a cation exchange resin slowly enough to allow equilibration, said metal salt being selected from the group consisting of copper (II) salt, nickel (II) salt, lead (II) salt, iron (III) salt and mixtures thereof; and collecting an effluent in fractions.

References Cited in the file of this patent UNITED STATES PATENTS Spedding et al. Jan. 23, 1951 OTHER REFERENCES 

1.A PROCESS OF SEPARATING A PLURALITY OF RARE EARTH METAL VALUES CONTAINED IN AN AQUEOUS SOLUTION FROM EACH OTHER, COMPRISING CONTACTING SAID SSOLUTION WITH A NONACIDIC CATION EXCHANGE RESIN WHEREBY SAID RARE EARTH METAL VALUES ARE ADSORBED ON SAID RESIN; CONTACTING SAID RESIN WITH AN AQUEOUS SOLUTION OF AN EHTYLENEDIAMINE TETRAACETIC ACID AGENT HAVING A PH VALUE OF AT LEAST 5 FOR A SUFFICIENTLY LONG PERIOD OF TIME TO ALLOW EQUILIBRATION, THE AMOUNT OF SAID ETHYLENEDIAMINE TETRAACETIC ACID AGENT BEING LESS THAN THE STOICHIOMETRIC AMOUNT REQUIREDD TO COMPLEX ALL OF THE RARE EARTHS ADSORBED ON SAID RESIN; AND SEPARATING THE RESIN CONTAINING THE NONCOMPLEXED RARE EARTH VALUES FROM THE SOLUTION CONTAINING THE COMPLEXED RARE EARTH VALUES. 